Electron image storage device



1963 J. F. NICHOLSON ELECTRON IMAGE STORAGE DEVICE Filed April 25, 1966 m|O xON O. m m..O xN o m cm d w 585m @258 0. mm 805592. :3. 2 am Eummnu coed can mdmwm .n w 1 of 3 w mm *2? 9 on m Nm on m 0E ATTORNEY United States Patent 0 3,406,311 ELECTRON IMAGE STORAGE DEVICE James F. Nicholson, Pine City, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 25, 1966, Ser. No. 545,049 7 Claims. (Cl. 315-12) ABSTRACT OF THE DISCLOSURE This invention relates to a storage device whichillustratively includes a photocathode element for emitting an electron image in response to an incident radiation image and for emitting a flow of electrons of substantially uniform distribution, a member having an aperture therein, an electron multiplier disposed remotely with respect to the photocathode to receive and multiply those electrons directed through the aperture, and a foraminated target for storing a pattern of charges in response to the electron image and for modulating the uniform flow of electrons in accordance with the pattern of charges.

This invention relates to electron image devices, and more particularly to those devices having the capability of storing electron images and of deriving at a later time a signal corresponding to the stored information.

Storage devices are well known in the art and typically include a storage target member capable of storing a pattern of electrical charges corresponding to the information to be stored. The information is placed or written upon the target member by converting a light image as by a photocathode element into an electron image and directing the electron image onto the target member. After a desired period of time, the information so stored may be taken off or read from the target member by directing a low velocity beam of electrons onto the storage target and an output signal is derived through either the conductive backplate of the target member or by collecting a return beam of electrons. In either method of obtaining the output signal, it is necessary to direct a reading beam of electrons onto the target member as by an electron gun. Such electron guns typically include a thermally emissive cathode element for generating electrons, focusing electrodes for defining the electrons into a beam and means such as electrostatic plates for deflecting the electron beam across the surface of the target member.

There are several significant disadvantages to such a storage system. First, an electron gun for producing the reading beam of electrons requires substantial amounts of power to excite as by a heater element the thermal emissive cathode element. Such power requirements would make such a storage system impractical for most space applications. Secondly, the gain of many storage target members is inherently low thereby limiting the overall efficiency of such a system. Finally, the storage target, which typically includes a conductor member upon which there is disposed a layer of dielectric storage material, has inherent limitations as to the resolution of the image stored thereon; more specifically, the charged pattern stored upon the dielectric material tends to dissipate through this layer to the conductive member thereby tending to blur the elemental charges of the stored pattern.

It is accordingly an object of this invention to provide an improved image storage device.

It is another object of this invention to provide an improved image storage device having substantially increased resolution.

A still further object of this invention is to provide an improved image storage device having significantly reduced power requirements.

3,406,311 Patented Oct. 15, 1968 More specifically, it is an object of this invention to provide an improved storage image device which avoids the necessity of using a thermionic emitting cathode for generating and projecting a reading electron beam onto a storage target member.

It is a further object of this invention to provide an improved storage image device having an extremely high gain with an inherently low noise factor.

A further object of this invention is to provide an improved storage image device having a high sensitivity to radiation and in addition being capable of integrating the radiation sensed over considerable periodsfiof. time.

It is a still further object of this invention to provide an improved image storage device capable of repeatedly reading out the stored image.

A further object of this invention is to provide an improved storage image device capable of also operating as an image camera device.

Briefly, the present invention accomplishes the above cited objects by providing an improved storage image device wherein the radiation to be sensed is directed upon a photocathode element which emits an electron image corresponding to the radiation image. The electron image is directed upon a foraminated storage member capable of storing a charge pattern corresponding to the electron image. Upon the remote side of the storage member with respect to the photocathode element, there is disposed a member having an aperture therein and a means for receiving and multiplying the electrons that are directed through the aperture. Thus, in operation, a pattern of charges may be stored upon the target member in response to radiation directed upon the photocathode element and the information may be read out by directing uniform radiation onto the photocathode element to provide a corresponding uniform electron image which is directed towards the storage target member. The uniform electron image is modulated in accordance with the pattern of charges stored upon the storage target member. After modulation by the target member, the modulated electron image is scanned by suitable means such as deflection coils across the aperture member with a portion of the modulated electron image being directed through the aperture to be received by the means for multiplying electrons.

A further aspect of this invention provides that by adjusting the focusing voltages upon the various electrodes of this device and upon the storage target member, that the storage image device may be used as a camera image device by directly scanning the electron image from the photocathode element over the apertured member in a manner similar to that used in those devices known as image dissectors.

These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in Which:

FIGURE 1 shows a sectioned view of a detailed embodiment of the image storage device in accordance with the teachings of this invention;

FIG. 2 shows a detailed view of the storage target which has been incorporated in the image storage device of FIG. 1;

FIG. 3 shows in graphical form a typical set of transfer characteristics of the storage image device of FIG. 1; and

FIG. 4 shows a diagrammatic view of the image storage device of FIG. 1 in which the various voltage sources and switching means are shown in order to more fully explain the operation of this device.

Referring now to the drawings and in particular to FIG. 1, an electron image device 10 constructed in accordance with the teachings of this invention includes an evacuated envelope 12 having an enlarged portion 14 and a concentrically aligned, elongated portion 16. The enlarged portion 14 of the envelope 12 is enclosed by a radiation transmissive face plate 18 upon which there is disposed a photocathode element 20. In an exemplary embodiment of this invention, the photocathode element 20 may be formed by first evaporating a layer of antimony upon the face plate 18 and then reacting this layer of antimony with appropriate alkali vapors.

There is disposed within the enlarged portion 14 of envelope 12 a focusing electrode 22, a target support member 26, and a focusing electrode 24. Typically, these electrodes are formed of a cylindrical configuration and are aligned centrally of the envelope 12 in the order enumerated. Illustratively, the electrodes 22, 24 and 26 are mounted upon support rods 30 made of an insulating material such as alumina or glass. The support rods 30 are connected to the respective electrodes by tabs 28 which may be welded to the exterior'surfaces of the electrodes.

The support rods 30 resemble the well known insulating spaghetti and connecting pins 32 may be inserted therethrough to provide appropriate electrical connections to the electrodes. The connecting pins 32 are embedded through a shoulder portion 38 of the envelope 12, which is formed between the enlarged portion 14 and the elongated portion 16, to provide external terminals. An electrical connection is provided for the photocathode element by a terminal foil 34 which has been formed over a portion of the photocathode element 20 and the inner surface of the enlarged portion 14. Further, a resilient contact 36 is secured to one of the connecting pins 32 so as to resiliently abut against the terminal foil 34.

Within the elongated portion 16 of the envelope 12, there is disposed a focusing electrode in the form of a conductive layer deposited upon the interior surface of the portion 14 for focusing the electron image emanating from the photocathode element 20 onto an aperture plate 48. The aperture plate 48 has an aperture therein to allow a portion of the electron image emitted by the photocathode element 20 to be directed therethrough. The aperture plate 48 is supported within the elongated portion 14 upon one end of a cup shaped electrode 46. In addition, an electron multiplier assembly 40 is disposed within the cup shaped electrode 46 to receive those electrons directed through the aperture 50. The electron multiplier assembly 40 includes a plurality of dynode elements 42 which are serially related with each other to successively multiply the electrons and to direct the multiplied electrons onto a dynode anode 44 from which the output signal of this device 10 is derived. The end of the envelope 12 opposite the electron multiplier assembly 40 is enclosed by an end plate 52 through which there are inserted a plurality of terminal elements 54.

It is noted that the device as so far illustratively de scribed resembles very closely that electron image device typically known as an image dissector. Such an image dissector is more fully described in a copending application of Mr. James F. Nicholson, entitled Television Camera Devices and Related Systems, filed July 17, 1964, Ser. No. 383,316, now US. Patent No. 3,341,734 and assigned to the assignee of this invention.

In accordance with the teachings of this invention, a storage target or member is disposed (as shown in FIG. 1) between the photocathode element 20- and the aperture plate 48 and is supported upon the member 26 as by an annular, cup-shaped flange 56. As shown in detail in FIG. 2, the storage target 60 includes an annular support member 62, a retainer ring 64 and a storage screen 66 with is stretched across and secured to a support ring 68. Further, a collector screen 72 is supported upon a support ring 74 and is spaced and insulated from the storage screen 66 by a ring 70 made of a suitable insulating material such as ceramic. The assembly of the storage screen 66 and the collector screen 72 is held together as by the retainer ring 64 which has a flange which abuts against an extended portion of the collector screen 72 to thereby secure together the various support rings of the storage target 60. Further, the extended portion of the collector screen 72 is coated with an insulating layer 76 to thereby insulate the collector screen 72 and the support ring 74 from the retainer ring 64. In order to make electrical contact with the collector screen 72, a ribbon contact 78 is secured to the support ring 74. Electrical contact may be made to the storage screen 66 as through the annular support member 62 and the retainer ring 64. The storage screen 66 is insulated from and is spaced a distance of approximately .0020 inch from the collector screen 72 by the spacer ring 70. Typically, the storage screen 66 is made of a conductive mesh of approximately 750 lines. per inch and is coated (see FIG. 4) with a layer 67 of a suitable storage dielectric material such as magnesium chloride magnesium oxide, or aluminum oxide. The collector screen 72 is likewise made of a conductive mesh with approximately 200 lines per inch which is of a coarser mesh than used to construct the storage screen 66.

Referring now to FIG. 4, there is shown the electron image device 10 in diagrammatic form in order to fully explain the operation of this device. A radiation image such as light corresponding to a scene 58 is focused by an optical lens system 59 onto the photocathode element 20 of the electron image device 10. A magnetic focusing coil 88 is disposed about the envelope 12 in order to focus the electron image onto the storage mesh 66 and the aperture plate 48. Further, the electron image emitted from photocathode element 20 and modulated by the pattern of charges stored upon the target member 60 is scanned in a pattern or raster over the plate 48 by a set of deflection coils 86. A shutter element 80 is disposed between the scene 58 and the photocathode element 20 in order to selectively allow the radiation to fall upon the photocathode element 20. More specifically, the shutter element 80 includes a movable element 82 which may be disposed in a first (or closed) position or in a second (or open) position. A light source 84 is disposed adjacent the photocathode element 20 in order to direct radiation of uniform intensity over the surface of the photocathode element 20. Further, the photocathode element 20 is connected externally of the envelope 12 by a switch means to either ground (i.e. position 1), to a potential source (i.e. position 2) or a potential source 96 (i.e. position 3). The focusing electrode 22 is connected externally of the envelope 12 by a switch means 91 to either ground (i.e. position 1), a potential source 98 (i.e. position 2) or to the potential source 96 (i.e. position 3). The collector screen 72 is connected externally of the envelope 12' by a switching means 93 to either a potential source 104 (i.e. position 1), to ground (i.e. position 2), or to a potential source 106 (i.e. position 3). Further, the electrically con ductive storage screen 66 is connected externally of the envelope 12 by a switching means 92 to either a potential source 100 (i.e. position 1), to a potential source 101 (i.e. position 2), or to a potential source 102 (i.e. position 3). It is noted that the various elements of the electron image device 10 are shown in greater detail in FIG. 1.

The operation of the electron image device 10 of the present invention will be explained with reference to FIG. 4 and may be considered to take place in the following three steps: priming, writing and reading.

PRIMING In order to prime the storage target 60, the switching means 90, 91, 92 and 93 are respectively disposed in their first position. Thus, in this illustrative mode of operation, the photocathode element 20 and the focusing electrode 22 are both placed at ground or substantially zero potential. With the movable element 82 of the shutter element 80 in the first or closed position, the light source 84 is energized to direct radiation of substantially uniform in tensity over the photocathode element 20. In response to the substantially uniform radiation, the photocathode element 20 emits a photoelectron image with a substantially uniform distribution. The photoelectron image is accelerated by a suitable potential such as +90 volts which is applied by the potential source 104 to the collector screen 72. The accelerated photoelectron image is directed onto the storage screen 66 by a suitable potential below the first crossover of the storage dielectric material 67 such as volts which is applied by the potential source 100 to the storage screen 66. Under these illustrative conditions, the dielectric storage material 67 will emit secondary electrons which are collected by the collector screen 72 thereby driving the surface of the layer 67 of the dielectric storage material to the approximate potential of that of the photocathode element 20. Therefore, at the end of the priming operation, the layer 67 of dielectric storage material will have a potential difference disposed thereacross of approximately 10 volts with the surface of the layer 67 disposed at substantially ground potential.

WRITING In order to write or to place a pattern of charges corresponding to the information to be stored upon the target member 60, the switches 90, 91, 92 and 93 are disposed respectively in their second position. Further, the movable element 82 of the shutter element 80 is disposed in its open or second position to thereby allow the radiation from the scene 58 to be focused as by the optical lens system 59 onto the photocathode element 20. It is noted that during this operation, that the light source 84 remains unenergized. In response to the radiation image, the photocathode element emits a photoelectron image having a spacial distribution which corresponds to that of the radiation image. The storage screen 66 of the storage target 60 is maintained at a positive potential with respect to the photocathode element 20 in excess of the first crossover potential of the dielectric storage material 67. Illustratively, the photocathode element 20 may be maintained at a negative potential of approximately 450 volts as by the potential source 95. In order to provide an appropriate focusing voltage upon the photoelectron image emitted by the photocathode element 20, a negative potential of approximately 350 volts is applied to the focusing electrode 22 as by the potential source 98. Further, the collector screen 72 is connected as by the switching means 93 to ground to establish the collector screen at approximately zero or ground potential. The storage screen 66 is disposed at a potential of approximately 3 volts positive with respect to ground by the potential source 101. As was noted in the description of the preceding operation, there was established a voltage difference across the layer 67 of the storage dielectric material of approximately 10 volts; thus, by establishing the storage screen 66 at approximately 3 volts positive, the surface of the layer 67 of storage dielectric material is disposed at approximately -7 volts. Thus, when the photoelectron image is accelerated with a potential above that of the first crossover of the dielectric storage material, the layer 67 will emit more secondary electrons than incident primary electrons to thereby drive the surface of the layer 67 of storage dielectric material towards the potential of the collector screen 72. Thus, the greater the density of a particular portion of the photoelectron image, the greater the number of secondary electrons will be emitted from the layer 67 and the closer will that portion of the layer 67 be driven to ground potential (i.e. the potential of the collector screen 72). As a result, various portions of the layer of the dielectric storage material 67 will be disposed at various potentials between approximately 7 volts corresponding to little or no radiation, and approximately zero volts corresponding to elemental portions of intense radiation.

READING In order to readout or to derive a signal corresponding to the pattern of charges stored upon the storage target 60, the switch means 90, 91, 92 and 93 are respectively disposed in their third position. Further, the movable element 82 of the shutter element is disposed in its closed or first position, and the light source 84 is energized to direct radiation of a substantially uniform intensity over the photocathode element 20. In response to the uniform radiation, the photocathode element 20 emits a photoelectron image with substantially uniform density which is focused as by the focusing electrode 22 in the plane of the aperture plate 48. Illustratively, the photocathode element 20 and the focus ing electrode 22 are maintained at the same negative potential of approximately 475 volts as by the potential source 96. The collector screen 72 is maintained at a suitable negative potential of approximately 380 volts as by the potential source 106, and the storage screen 66 is disposed at a suitable negative potential. of approxia mately 470 volts as by the potential source 102. As noted above, the storage target 60 is foraminated thereby allowing a portion of the photoelectron image to pass therethrough. With a negative potential of approximately 470 volts applied to the storage screen 66, various portions of the layer 67 of dielectric storage material will be disposed at potentials varying from a negative potential of approximately 480 volts (corresponding to a low intensity of radiation) to a negative potential of approxi mately 473 volts (corresponding to intense radiation). As the uniform photoelectron image is directed through the foraminated storage screen 66, the electron image is modulated as by the pattern of charges stored upon the dielectric storage material 67 in a manner very similar to that of the control grid in a conventional receiving electron tube. The modulated electron beam is then directed onto the aperture plate 48 where a portion of the modulated electron image passes through the aperture 50 to be multiplied by the assembly 40 and collected by the dynode anode 44 to derive an output signal of the electron image device 10. The deflection coil 86 acts to deflect the modulated electron image derived from the storage target 60 in a raster or pattern across the surface of the aperture plate 48. Thus, successive portions of the information stored upon the target 60 will provide the output signal derived from the anode 44. It is noted from the illustrative method of operation that the reading photoelectron image does not land upon the layer 67 of dielectric storage material. Therefore, there is substantially no dissipation of the pattern of charges stored upon the target 60 and a non-destruction readout is achieved whereby the information to be stored may be viewed as upon a television monitor over a long period of time.

One of the significant advantages of the storage image device of this invention is the inherent high gain of this device and this target element. Generally, very low levels of light intensity may be sensed by the storage image device. This advantage is a result of the high resistivity of storage dielectric material of the layer 67 which allows the low intensities of light and the corresponding photoelectrons to be integrated over long periods of time to thereby build up the desired intensity of the pattern of charges. On the other hand, the readout operation may be achieved with a light source of high intensity thereby deriving a relatively large output signal. In an illustrative mode of operation, a current in the order of 10" to 10 amperes was required to write the pattern of charges upon the storage target 60. During the reading operation, a light source 84 of approximately .5 lumens is directed onto a photocathode element 20 having a sensitivity of approximately 200 microamps per lumen thus providing a photoelectron flood current of approximately microamperes. Thus for the ideal situation with 100% modulation contrast, a write-read gain would be achieved of:

I, 100 X 10' amps Thus, without taking into consideration the transmission of the screens 56 and 72, an ideal gain of approximately 10 could be calculated.

Referring now to FIG. 3, an actual graph of the transfer characteristics for a storage image device identified by the number WX-31282 by the assignee of this invention is presented in graphical form. To obtain this graph, the photocathode element was illuminated with a light source of approximately 5 x 10- foot candles during the reading operation. Thus, from the graph presented in FIG. 3, it could be determined that a scene having an intensity of 10 x 10* foot candles and integrated for approximately one second would provide an output of 4 microamperes in that instance where the photocathode was excited with 10 microamperes during the readouts, thus, an actual gain of approximately 4 X 10 would be derived.

A further advantage of the storage image device of this invention is that this device does not use a heater element for a thermionic cathode. Thus, this device requires substantially less power than other storage devices. In illustrative terms, the photocathode element of this invention requires approximately .05 micro-watts, Whereas a typical heater element for the cathode element of a low velocity writing electron beam requires 3.6 watts. Further, there is no target blooming or beam bending that occurs with image tubes that do require a low velocity electron gun with a thermionic cathode element. In addition, due to the construction of the storage target 60 as described with respect to FIG. 2, significantly larger target elements may be used in this device. Specifically, the target elements of such devices as an image orthicon camera tube have been limited to approximately one inch, whereas the storage target of this invention may have a diameter of up to approximately 12 inches thereby providing substantially increased resolution to the stored image. Finally, the storage image device of this invention may be used either as a storage device or as a typical television camera tube in the manner of an ordinary image dissector.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description. or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. An electron image device for sensing a radiation image comprising first means for emitting an electron image corresponding to said radiation image and for emitting a flow of electrons of substantially uniform density, second means for storing a pattern of charges corresponding to said electron image and for modulat ing said flow of electrons in accordance with said pattern of charges, a member disposed to receive said modulated flow of electrons and having at least one aperture therein, third means for collecting that portion of said modulated flow of electrons directed through said aperture, and fourth means for scanning said modulated flow of electrons across said aperture.

2. A storage system including said electron image device as claimed in claim 1, wherein there is included means for selectively directing said radiation image onto said means for emitting electrons, and means for directin-g radiation of substantially uniform intensity onto said means for emitting electrons.

3. An electron image device as claimed in claim 1, wherein said second means for storing includes a forminated layer of electrically conductive material; and a foraminated dielectric layer disposed upon said layer of electrically conductive material and having the property of establishing said pattern of charges in response to electron bombardment.

4. An electron image device as claimed in claim 1,

wherein said second means for storing includes a first electrically conductive, foraminated layer; a second dielectric layer disposed upon said zfirst layer having the property of establishing said pattern of charges in response to electron bombardment; and a third foraminated layer of electrically conductive material disposed between said first means and said first electrically conductive layer.

5. A storage system including said electron image device as claimed in claim 1, wherein said first means include a photocathode element for emitting electrons in response to incident radiation, and said storage system further include shutter means selectively permitting said radiation image to be directed onto said photocathode element whereby said photocathode element emits said electron image corresponding to said radiation image; said second means for storing including an electrically conductive member having a plurality of apertures therein, a dielectric layer disposed upon said electrically conductive member and having the property of establishing said pattern of charges corresponding to said radiation image in response to the bombardment of said first electron image; and a source for directing radiation of substantially uniform intensity onto said photocathode element to provide said flow of electrons of substantially uniform density which is to be directed through said apertures of said first member and to be modulated in accordance with said pattern of charges.

6. A storage system including said electron image device as claimed in claim 1, wherein said first means includes a photocathode element for emitting electrons in response to incident radiation, and said storage system further includes shutter means for selectively permitting said radiation image to be directed onto said photocathode element whereby said photocathode element emits said electron image corresponding to said radiation image; said first means for storing includes a first electrically conductive member having a plurality of apertures therein, a second dielectric layer disposed upon said first layer and having the property of estab lishing said pattern of charges corresponding to said radiation image in response to the bombardment of said electron image, and a third electrically conductive member having a plurality of apertures therein and disposed between said photocathode element and said first electrically conductive member, a source for directing radiation of substantially uniform intensity onto said photocathode element whereby said photocathode element emits said flow of electrons of substantially uniform density which is to be directed through said apertures of said first member and to be modulated in accordance with said pattern of charges, said fourth electrically conductive member with respect to said photocathode element to collect and to multiply said portion to thereby provide an output signal.

7. An electron image device including first means for emitting an electron image in response to a radiation image, second means for storing a pattern of charges in response to electron bombardment and for modulating a flow of electrons in accordance with said pattern of charges, a member disposed to receive said modulated flow of electrons having at least one aperture therein, and third means for collecting that portion of said modulated flow of electrons directed through said aperture.

References Cited UNITED STATES PATENTS 2,981,863 4/1961 Schneeberger et al. 31512 3,277,334 10/1966 Toohig et al. 315-12 3,281,621 10/1966 Clayton 315-12 3,304,462 2/1967 Koda 315-12 RODNEY D. BENNETT, Primary Examiner. I. P. MORRIS, Assistant Examiner. 

